Abstract

An important goal in ecology is to understand controls on community structure in spatially and temporally heterogeneous landscapes, a challenge for which riverine floodplains provide ideal laboratories. We evaluated how spatial position, local habitat features, and seasonal flooding interact to shape aquatic invertebrate community composition in an unregulated riverine floodplain in western Alabama (USA). We quantified sediment invertebrate assemblages and habitat variables at 23 sites over a 15-month period. Dissolved oxygen (DO) varied seasonally and among habitats, with sites less connected to the river channel experiencing frequent hypoxia (<2 mg O2 L−1) at the sediment–water interface. Differences in water temperature among sites were lowest (<1 °C) during winter floodplain inundation, but increased to >14 °C during spring and summer as sites became isolated. Overall, local habitat conditions were more important in explaining patterns in assemblage structure than was spatial position in the floodplain (e.g., distance to the main river channel). DO was an important predictor of taxonomic richness among sites, which was highest where hydrologic connections to the main river channel were strongest. Compositional heterogeneity across the floodplain was lowest immediately following inundation and increased as individual sites became hydrologically isolated. Our results illustrate how geomorphic structure and seasonal flooding interact to shape floodplain aquatic assemblages. The flood pulse of lowland rivers influences biodiversity through effects of connectivity on hydrologic flushing in different floodplain habitats, which may prevent the development of harsh environmental conditions that exclude certain taxa. Such interactions highlight the ongoing consequences of river regulation for taxonomically diverse floodplain ecosystems.

Notes

Acknowledgments

We would like to thank Elise Chapman, Mike Dewar, Michael Kendrick, Reed Morgan, Whit Morgan, James Ramsey, and Michael Venarsky for their help in the laboratory and field. Constructive comments by Alex Huryn and two anonymous reviewers improved the quality of the manuscript. Alabama State Lands Division and Forever Wild Land Trust provided access to the Sipsey River Swamp Recreation Area and Nature Preserve. The project was supported by a grant from the University of Alabama (UA) Research Grant Council to R. Sponseller, by the UA Department of Biological Sciences, and by an Ilouise Hill Research Fellowship to S. Starr. The LiDAR survey was funded through a National Center for Airborne Laser Mapping (NCALM) seed proposal.

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